1 Field of the Invention
The invention relates to an electronic switching device, preferably with a proximity indicator, for example, an oscillator, and with a an electronic switch which can be controlled by a proximity indicator, optionally via a switching amplifier. Furthermore, the invention relates to a circuit for monitoring of the satisfactory state and/or detection of a faulty state of a system, for example, a aerial railway or chairlift system, with a plurality of monitoring and detection devices and with at least one control and evaluation unit, the monitoring and detection devices being made as electronic switching devices of the aforementioned type and being series connected to one another and to the control and evaluation unit.
2 Description of Related Art
Electronic switching devices of the type to which the invention is directed are made of solid-state construction and have been increasingly used for almost thirty years instead of mechanically activated, electrical switching devices which have contacts, especially in electrical or electronic measurement and control circuits. In a contact-type switching device, either an electronic switch which was initially nonconductive becomes conductive (make contact type), an electronic switch which was initially conductive becomes nonconductive or blocked (break contact). This applies particularly to proximity switches, i.e., to proximity electronic switching devices.
With proximity switching devices, it is indicated whether an influence element to which the corresponding proximity switch is sensitive has approached near enough to the proximity switch. If, specifically, the influence element to which the corresponding proximity switch is sensitive has approached the proximity switch closely enough, the proximity indicator switches the electronic switch. With switching devices of the type under consideration, it can also be indicated whether a physical quantity of an influencing medium to which the switching device is sensitive has reached a corresponding value.
Thus, one important component of electronic switching devices of the type under consideration is, among others, the proximity indicator which can be externally influenced. In inductive or capacitive proximity switches, the proximity indicator is generally an inductively coupled oscillator or a capacitively coupled oscillator. Optoelectronic proximity switches have a photoresistor, a photodiode, or a phototransistor as the proximity indicator. Otherwise, electronic switching devices of the type to which the invention is directed include, for example, also so-called flow indicators which have a temperature measurement circuit.
Electronic switching devices of the type under consideration are often made as so-called two-wire switching devices, i.e., they can be connected via an external lead to one pole of a voltage source and only via another external lead to one terminal of a consumer, the other terminal of the consumer being connectable to the other pole of the voltage source.
In electronic switching devices which can be connected only via one external lead to one pole of a voltage source and only via one other external lead to one terminal of a consumer, it is not a problem to make available the operating voltage (=internal operating voltage) or the operating current required by the proximity indicator, and optionally, also the switching amplifier. This is because the internal operating voltage or operating current must be made available both in the conductive and also in the blocked state.
It is irrelevant whether an operating current or an internal operating voltage is made available, since the proximity indicator, and optionally the switching amplifier, of course require electrical power. Therefore, both an internal operating voltage and also operating current are required (compare the representation of these circumstances in German patent document 23 30 233, especially in column 5, line 68, to column 6, line 33).
Based on their function as switching devices, in the switching devices under consideration, in the conductive state almost no voltage drop will occur and in the blocked state almost no residual current will flow. However, it applies to all two-wire switching devices that, in the conductive state a voltage drop occurs and in the blocked state a residual current flows since, if no voltage drop were to occur in the two-wire switching devices in the conductive state, since no internal operating voltage for the proximity indicator and optionally the switching amplifier could be obtained, if in the blocked state no residual current were to flow, nor could any operating current be obtained.
It has already been stated initially that electronic switching devices of the type under consideration are used in measurement and control circuits. In particular, a plurality of switching devices of the type under consideration are often required in control circuits. The individual switching devices can be located relatively far apart and relatively great distances can be bridged between the individual switching devices and a central control, so that wiring of the switching devices among one another and to a central control can be expensive.
It has also been already stated initially that electronic switching devices of the type under consideration can be made as make contacts or a break contacts, In safety-relevant control circuits, switching devices made as break contacts are used since the safe state must be the de-energized state. The use of switching devices of the type under consideration made as break contacts, in safety relevant control circuits, requires series connection of all switching devices. If, in this series connection, only one switching device made as a break contact is not influenced, the electronic switch to which this switching device belongs is blocking, the circuit formed by series connection of all of the switching devices is de-energized.
As noted above, on switching devices which are made as two-wire switching devices, a voltage drop also occurs in the conductive state, as necessary for operation; this voltage drop can be roughly 5-10 V or even more throughout. In the series connection of a plurality of switching devices of the type under consideration, the voltage drop which occurs throughout the series connection of these switching devices can easily assume a value which is no longer acceptable.
Electronic switching devices of the type under consideration are made of a solid-state construction, as noted initially. Instead of the contact provided in an electrical switching device, there is an electronic switch. Even in those electronic switching devices of the type under consideration which are made not as two-wire switching devices, but as three-wire switching devices, in the conductive state, a voltage drop occurs which is not negligible. This voltage drop can also be added to a voltage drop which is no longer acceptable in the series connection of a plurality of switching devices of the type under consideration.
Systems of the most varied type, especially so-called safety-relevant systems, often require continual monitoring, i.e., monitoring of the satisfactory state, and generally in conjunction with monitoring of the satisfactory state, the detection of a faulty state. As soon as a faulty state has been detected, a safety-relevant measure is initiated.
In aerial railway and chairlift systems, the location of the cable relative to the cable rollers guiding the cable or carrying the cable must be monitored as the safety-relevant criterion. If the actual position of the cable relative to at least one cable roller does not correspond to the theoretical position, safety-relevant measures must be taken. For a slight deviation between the actual position of the cable and its theoretical position, the safety-relevant measure derived therefrom can be a reduction of cable speed. If the deviation between the actual position of the cable and the theoretical position exceeds a stipulated boundary value, the safety-relevant measure consists, in any case, in that the aerial railway system or chairlift system is instantaneously shut down.
In electrical, and today of course also in electronic circuits which act as control circuits, a distinction is made between the open circuit principle and the closed circuit principle. In the open circuit principle, for example, there is a control intervention into a system, for example, when a large enough control current flows in the control circuit. Switching devices in control circuits operating according to the open circuit principle are made as make contacts; they are electrically conductive when they are actuated. In contrast, in the closed circuit principle, for example, there is a control intervention into the system when no control current flows in the control circuit. Switching devices in control circuits operating according to the closed circuit principle are made as break contacts; they are electrically nonconductive when they are actuated.
In control circuits operating according to the open circuit principle, as was mentioned above, there is a control intervention into an assigned system when a sufficiently large control current flows in the control circuit. Therefore, the prerequisite for this is that, on the one hand, the necessary operating voltage is present, on the other hand, that there is no line break, and finally, that the switching device which is made as a make contact and which is present in this control circuit is serviceable. In contrast, in a control circuit which operates according to the closed circuit principle, there is a control intervention into a system belonging to it if no control current flows in the control circuit.
The aforementioned statements on the open circuit principle and the closed circuit principle make it clear that control circuits which are designed to trigger safety-relevant measures must operate according to the closed circuit principle. Since the de-energized state in the control circuit is evaluated such that the corresponding system is in the safe state, a failure of the operating voltage and a line break always lead to the system, if it is not in the safe state, being moved into the safe state. In an aerial railway or in a chairlift system, the safe state is of course for the cable drive to be turned off.
In control circuits operating according to the open circuit principle, therefore, in control circuits in which the switching devices are made as make contacts, the switching devices are connected in parallel; after actuating a switching device made as a make contact, the control circuit becomes active again. Conversely, in control circuits operating according to the closed circuit principle, therefore, in control circuits in which the switching devices are made as break contacts, the switching devices are connected in series, after actuating a switching device made as a break contact, the control circuit becomes de-energized and thus entirely active.